The present invention is directed to fuel cells, and more particularly to a fuel cell system employing at least one electrokinetic pump for moving fuel.
There are many different types of fuel cells currently in operation. Some fuel cells operate using gasses and some operate using liquids. The present invention is directed to liquid fuel cells. There are many varieties of liquid fuel cells, including methanol reacting fuel cells. The popularity of methanol results from the fact that it has a very high proton density. However, many other liquid fuels have been suggested, ranging from formic acid to sugar water. The present invention is applicable to liquid fuel cells in general.
One mode of fuel cell operation is the direct catalytic conversion method. Methanol based fuel cells employing this method are known as DMFC, or direct methanol fuel cells. The methanol is delivered directly to a membrane-electrode assembly (“MEA”) with appropriate catalytic materials to extract protons from the fuel directly. The protons are transferred across the membrane to a catalytic electrode where the protons are converted to water by reaction with an oxidant, typically oxygen.
Another mode of fuel cell operation involves thermocatalytic degradation of a liquid fuel to generate hydrogen gas. The hydrogen gas is then sent to a membrane-electrode assembly where the hydrogen catalytically decomposed. The protons pass through the membrane and are oxidized to water on a second catalytic electrode. In both methods, the electrons are transported from one side of a membrane-electrode assembly to the other side of the membrane-electrode assembly through an electrical circuit where the displacement current used to power electrical devices.
One major problem with fuel cells has been the lack of an adequate solution for delivery of liquid fuel. Initially, efforts were made to develop passive delivery systems, such as systems employing capillary forces, diffusion, etc., but these delivery systems did not provide enough fuel to meet the electricity generation requirements of current demanding applications such as portable electronics. Currently, fuel delivery is often done by a diaphragm pump using a microfabricated diaphragm and check valves and a piezo electric stack for actuation. This type of pump has several drawbacks.
Typically, the pump operates at very high voltages, requiring specialized electronic circuitry for operation. Additionally, existing pumps typically operate in a half-rectified mode having only a 50% duty cycle, thereby leading to pulsation in the delivery of the fuel. Moreover, the volume displacement per cycle is typically very small leading to inefficiency in pumping. To mitigate this problem, pump developers tend to operate the pumps at a very high frequency which removes fine control of the pump, makes the pump subject to reduced performance if bubbles are present, and often generates a buzzing sound. This also hinders the use of the device with a feedback control loop based upon the power output of the cell and/or the power requirements of the device. Therefore, there exists a need for an improved fuel cell pump.
Generally, existing systems simply pump a mixture of fuel from a reservoir through a chamber that contains the membrane-electrode assembly. One problem with this approach is that a significant portion of the fuel traverses the chamber without ever coming into contact with a catalyst in the membrane-electrode assembly. The depletion of the fuel occurs only at the membrane-electrode assembly. Without an active mechanism for transport of the fuel perpendicular to the surface of the membrane-electrode assembly, the only means for fuel to reach the membrane-electrode assembly is diffusion. This translates directly into an inefficiency of the overall fuel delivery system, because liquids are being transported through the system without effective energy conversion.
An additional problem is that fuel is expended as it traverses the length of the chamber, lowering the overall concentration as a function of location along the length of the membrane-electrode assembly. As fuel comes into contact with the membrane-electrode assembly, the fuel is converted into electrical current. With only diffusion to replenish fuel in the boundary layer of the flow, a gradient concentration distribution is established across the face of the membrane-electrode assembly, making the performance in some regions of the cell less efficient than others. There is therefore a need for improved fuel delivery within the fuel cell.